President Donald Trump’s approval rating is gradually climbing toward levels it has not reached since last spring, and along with it, Republican prospects for retaining control of Congress in November are rising, but doubts remain about the extent to which his presence on the campaign trail can offer an advantage for GOP candidates.

Tuesday night’s primaries brought victories for candidates Trump endorsed and the defeat of one he urged voters to reject. On Thursday, he was set to travel to Vice President Mike Pence’s home state of Indiana to rally with the new Republican Senate nominee there.

Still, Trump’s average approval rating now sits at 43 percent, according to RealClearPolitics, continuing steady improvement since it fell to the mid-30s in December and better than at nearly any point in the last year. The improved poll numbers come as he turns his attention toward promoting Republican candidates for the midterm elections.

In addition, voters’ opinion of Trump’s handling of several major issues is rising. On the economy, trade, foreign affairs, and immigration, his approval is up an average of 4 points since March.

The CNN poll also found Democrats’ one-time double-digit advantage in a generic ballot match-up with Republicans has slipped to only 3 points. Republican voter enthusiasm has ticked upward, with 44 percent saying they are “very enthusiastic” about voting in November.

“The challenge for the Republicans is that the president is an asset largely in safe congressional districts and may be a liability in contested congressional districts,” said Glenn Altschuler, a professor of American Studies at Cornell University. “Therefore, vis-a-vis Congress, he may be an asset in the districts where his presence is not needed.”

Trump’s support is weak in many places where Democrats aim to pick up House seats—California, New York, suburban Pennsylvania. On the Senate side, his popularity aligns better with the GOP’s needs.

“That’s where the Republican Party will try to make the best use of him, in West Virginia, in Indiana, in Missouri, perhaps in Montana where he’s already been critical of Sen. [Jon] Tester,” Altschuler said.

Trump’s Indiana rally Thursday follows several others in states he won in 2016 where competitive Senate races are expected this fall. Republicans are hoping he can help jumpstart campaigns against Democratic incumbents like Sen. Joe Donnelly (Ind.), Sen. Joe Manchin (W.V.), and Sen. Sherrod Brown (Ohio).

Democratic strategist Scott Ferson doubts Trump’s slightly improved popularity will shake up key races. In 2017 special elections, several Trump-endorsed candidates either lost or barely eked out victories in historically Republican districts. Most notably, in Alabama, his preferred candidate lost the Senate primary to Roy Moore, and he then supported Moore in the general election despite several allegations of sexual misconduct.

“If Roy Moore had won, then I think that would give Donald Trump a special power we didn’t know was otherwise there,” Ferson said. “Voters in Indiana and West Virginia know Donnelly and Manchin. It’s going to be a referendum on them.”

David Payne, a Republican strategist, expects Republican candidates’ fates will be more closely intertwined with Trump because the president has so thoroughly commandeered the country’s political discourse.

“Trump is going to play large in this year’s congressional elections whether the candidates want him to or not…,” he said. “He owns the national political tone. He either created the tone or it’s a reaction to him on the part of Democrats.”

According to the CNN poll, Trump is now as popular as President Jimmy Carter was at this point in his term, but he has spent much of his first 16 months in office as the least popular president on record. Given his unpredictable behavior and the speed with which news cycles have moved during his presidency, Republican candidates who embrace him now run the risk of his numbers tanking again around the election.

“Could he become a lead balloon at rallies across the country?” Payne said. “You bet, but he could also be an ally.”

Republicans interviewed on Capitol Hill Wednesday said they were not putting much stock in the outcome of any one primary, but they remain confident they can win their own races and their party can maintain control of the House.

“I’m still very optimistic. I think we’ve got a good shot,” said Rep. David Valadao, R-Calif., adding that he retained his seat in 2016 despite Clinton beating Trump in his district by 15 points.

“I think we’ve got major issues we can debate as a country about tax policy, economic growth and the lives of average Americans,” said Rep. Patrick McHenry, R-N.C. “I think we’ve got a good record to run on as Republicans and I believe I’ve got a good record as well to run on and receive another term.”

To retake control of the House, Democrats need to gain 23 seats in November. Based on special election results and polls, CNN has identified 73 GOP-held seats that could be competitive, and other experts have set the number of competitive races even higher. Democrats only need two seats to seize the majority in the Senate, but far fewer seats are realistically seen as in play.

Amid warnings of a blue wave washing out Republicans in the midterms, Tuesday’s primary results and the latest polls offer a reminder that November is very far away and nothing is guaranteed.

“In politics, six months is an eternity and it’s important for everyone to remember that,” Altschuler said. “So, for example, the euphoria of the Democrats over the last months must now be tempered by a recent poll showing that the generic preferences of Americans are closely divided between Democrats and Republicans.”

While the election is far off and the generic ballot becomes a less useful barometer the closer it gets, Payne said the trend-lines are important and they are clearly moving in Republicans’ direction.

“If that holds, Trump will be a credible and even useful endorser of Republican candidates,” he said.

Payne pointed to a shift in Trump’s rhetoric in recent months. He has scaled back belittling comments about GOP lawmakers like Senate Majority Leader Mitch McConnell, R-Ky., and Sen. Bob Corker, R-Tenn., focusing his attacks more on Democratic incumbents and candidates.

“He’s really held back on the friendly fire, and this has helped the party and him as an endorser of candidates and it’s made him a more reliable ally,” he said.

Payne also expects Pence, who will be in attendance at Thursday’s rally, to be a factor in energizing the Republican base this fall.

“I think Pence ends up being a secret weapon in the midterms…,” he said. “Trump’s support can help you and it can hurt you as a Republican. Pence’s support can only help you as a Republican this year.”

Still, Democrats have many reasons to remain optimistic, including a streak of strong performances and unexpected victories in special elections in traditionally Republican districts. Even when they have lost, their candidates outperformed 2016 presidential nominee Hillary Clinton’s vote share by double digits.

According to strategists, Trump’s improving poll numbers illustrate why Democrats must pitch themselves to voters as more than just anti-Trump. They need to be pro-something, and even some supporters feel they have struggled to articulate exactly what that is.

“The biggest issue is giving voters a reason to vote for them,” Ferson said. “We know why voters should vote against Donald Trump’s agenda, but we don’t know why we should vote for them. I don’t, and I’m a Democrat.”

Democratic leaders are aware of the challenge. At a Politico event Tuesday, House Minority Leader Nancy Pelosi, D-Calif., said she is urging her party’s candidates to talk about infrastructure, jobs, and rolling back corporate tax cuts.

“It comes down to an economic message,” she said. “The financial instability of American families is something that needs to be addressed.”

With several ethics scandals percolating around Trump’s Cabinet members and his lawyer, experts say an anti-corruption message similar to the one Trump ran on in 2016 may also resonate.

“I could see a Democratic campaign that emphasized corruption, that turned the ‘drain the swamp’ rhetoric to the Democrats, and argued for several reasons that a check on the administration is essential in this climate,” Altschuler said.

According to Democratic strategist Craig Varoga, bringing balance and accountability to Washington will be an effective theme for candidates, but it will not be enough.

“Democrats also need to lead in the election by offering a positive vision of what they plan to do next year, rather than over-relying on the fact that a majority of Americans disapprove of Trump’s performance in office,” Varoga said.

If claims of corruption evolve into cries of impeachment, Republicans predict it will backfire.

“If that remains their key message on the campaign trail, they’re playing right into Republican hands,” Payne said. “The tone will remain hysterical and feverish. If they’re smart, they’ll tone that down.”

As both parties strategize for November from six months out, Altschuler stressed that much can happen between now and then, both domestically and internationally, and some of the most impactful factors—like violence in the Middle East or a bombshell from special counsel Robert Mueller’s investigation—would be outside the control of the president or the candidates.

“Right now, it seems to me that the best advice to both Republicans and Democrats is to run scared,” Altschuler said, “because indeed there is no reason for confidence, let alone overconfidence, about the outcome of either house of Congress.”